This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Pyruvate:ferredoxin oxidoreductase (PFOR) exists as a dimer in Desulfovibrio africanus, but as an octomer (MW >1 MDa) in Desulfovibrio vulgaris. The question that this project will address is whether there are observable shifts in the helix packing when the two structures are compared at sub-nanometer resolution. The structural comparison of dimeric and octomeric complexes of this enzyme is expected to play an important role in understanding why it has proven to be biochemically more advantageous for D.vulgaris to assemble PFOR as an octomer while the dimeric form of the enzyme is better suited for D. africanus.